The present invention relates to delivery flow rate control apparatus that control the delivery flow rate of pumps installed in process piping systems such as a fossil fuel, nuclear and other types of power generation plants.
In general, many pumps are installed in process piping systems at power generation plants (hereinafter termed simply "plants") so as to send the process liquids under pressure.
These pumps usually control the flow rate so that the flow describes those operation limit conditions that relate to the pump delivery flow rate (or the suction flow rate).
a) Maximum allowable flow rate: This is the flow rate for which when a delivery flow rate (or a suction flow rate) greater than this flow rate flows through a pump, air bubbles are formed in the process liquid on the suction side of the pump and cause cavitation which may possibly destroy the pump and cause other problems such as a dramatic lowering of the pump delivery head (delivery pressure).
b) Minimum allowable flow rate: This is the flow rate for which when there is operation of the pump at a delivery flow rate lower than this, there is the possibility of a sharp increase in the temperature of the process liquid inside the pump and of the occurrence of trouble in the pump.
The present invention relates particularly to a) above, and controls the delivery flow rate of the pump while monitoring the maximum allowable flow rate.
FIG. 36 is a view of a conventional pump delivery flow control apparatus.
In the Figure, 1 represents a tank for the temporary storage of the process liquid, and the pressure of the process liquid that is stored in this tank is increased by two pumps 2.sub.-1, 2.sub.-2 which are arranged in parallel and which are respectively provided with drive apparatus 3.1, 3-2, and is sent to the process piping system via a flow rate adjuster valve 4 that controls the total delivery flow of the two pumps 2.sub.-1, 2.sub.-2.
The pump delivery flow rate control apparatus is provided with a flow rate meter 5 that detects the total delivery flow of the two pumps 2.sub.-1, 2.sub.-2, a flow rate adjuster 15e, an electro-pneumatic converter 13, and a flow rate adjuster valve 4 that is driven by pneumatic signals. Here, the flow rate adjuster 15e is configured from a flow rate deviation calculation portion 17 that outputs the deviation between the measured value (available flow rate value) from the flow rate meter 5 and the required flow rate value "a" from the side of the plant or the like, a PID calculation portion 8 that performs integral and differential calculation, a signal converter portion 12 that converts a valve degree of opening of the flow rate adjuster valve 4 into a predetermined fixed degree of opening value when one of the two pumps 2.sub.-1, 2.sub.-2 has failed and stopped.
Since it can be generally said that the pump delivery flow rate is equal to the pump suction flow rate, the flow rate meter 5 can be disposed on either the delivery side or the suction side of the two pumps 2.sub.-1, 2.sub.-2 but here, the description will be given in terms of when it is disposed on the delivery side of the pumps 2.sub.-1, 2.sub.-2.
The following is a description of the operation of a pump delivery flow rate control apparatus having the configuration described above.
The flow rate adjuster 15e has as its input the required flow rate set value "a" and the available flow rate value b measured by the flow rate meter 5 for the process liquid that is actually sent to the plant by the two pumps 2.sub.-1, 2.sub.-2. In the flow rate adjuster 15e, the flow rate deviation calculation portion 7 calculates the deviation between the available flow rate value b to the plant and the required flow rate set value "a" from the plant, and the PID calculation portion 8 outputs signals that have been given proportional, integral and differential calculation processing.
When there is normal operation, the required flow rate set value "a" from the plant is set so that it is smaller than the total value for the maximum allowable flow rate for the two pumps 2.sub.-1, 2.sub.-2, and in this case, the signals output from the PID calculation portion 8 are output as output signals from the flow rate adjuster 15e and via the signal converter portion 12. These output signals that are output from the flow rate adjuster 15e are converted into pneumatic signals at the electro-pneumatic converter 13 and are then input to the flow rate adjuster valve 4.
In this manner, the flow rate adjuster valve 4 performs open and close control by the pneumatic signals from the electro-pneumatic converter 13 so that the available flow rate of the process liquid to the plant is in agreement with the required flow rate set value "a" from the plant.
However, when there is this normal operation and either one (pump 2.sub.-2 for example) of the two pumps 2.sub.-1, 2.sub.-2 that are operating fails, the required flow rate set value "a" from the plant stays at that for two pumps and so the degree of opening of the flow rate adjuster valve 4 is maintained at the former degree of opening. Because of this, the delivery flow rate value of the pump that did not fail and stop (pump 2.sub.-1) increases to exceed the maximum allowable flow rate for that pump (pump 2.sub.-1) and generate trouble.
In addition, when the available flow rate value "b" to the plant, that is, the actual flow rate value for the pump (pump 2.sub.-1) does not become more than the required flow rate set value "a" from the plant (i.e. a&gt;b), the flow rate adjuster valve 4 operates so that the degree of opening of the flow rate adjuster valve 4 is further increased and there is the further likelihood of the occurrence of the trouble described above.
Also, depending upon the plant, when one of two operating pumps has failed and stopped, the method generally used to prevent the above described problems such as the generation of cavitation, the lowering of the pump delivery pressure and the like from occurring is to output a fixed value from the adjustment valve degree of opening setting portion 11 by the signal converter portion 12 and to monitor the required flow rate set value "a" from the plant so that the flow rate adjuster valve 4 is closed to a fixed, rated degree of opening that has been set before so that the delivery flow rate of the pump is brought to within the maximum allowable flow rate.
Furthermore, with this conventional technology, when one of two pumps that are operating fails and stops, the degree of opening of the flow rate adjuster valve 4 provided on the delivery side of the two pumps 2.sub.-1, 2.sub.-2 is decreased to a rated degree of opening set beforehand but another known method involves controlling the speed of one of the pumps that is operating (pump 2.sub.-1) so that the pump delivery flow rate is controlled. In this case, the degree of opening of the flow rate adjuster valve 4 is not necessarily decreased but a flow rate adjuster 15e (the same as described earlier for the conventional technology) is used so that it is possible to change the speed of the pump that did not fail (pump 2.sub.-1 for example), to a rated speed that has been set beforehand.
In addition, the description for this conventional technology has been for when the objective value for the speed of the pump or the objective value for the degree of opening of the flow rate adjuster valve 4 when one of the pumps has failed and stopped is a fixed value and for when there is immediately changed to this value when one of the pumps fails and stops. However, in certain cases, the general practice is to gradually change the value in steps so that it is ultimately made the predetermined fixed objective value for the speed of the pump or the objective value for the degree of opening of the flow rate adjuster valve 4.
Also, the above description for the conventional technology was for when there are two pumps disposed in parallel but when there are three or N number of pumps disposed in parallel, the number of operating pumps is detected and there is switching to the objective value for the speed of the pump or the objective value for the degree of opening of the flow rate adjuster valve and that is predetermined in accordance with that number (N-1, N-1 ,,, 2, 1) of pumps.
However, in this conventional case, even if a pump flow rate control apparatus is used as described above, it is not always possible to prevent the generation of trouble such as cavitation and in cases such as this, the general practice to prevent cavitation and the like is as described below.
More specifically, when the generation of cavitation commences, the suction pressure or the delivery pressure of the pump that is operating normally drops, and this is used to calculate beforehand the total delivery pressure or the total suction pressure (but the delivery pressure will be used for the description of the conventional technology) of the pumps 2.sub.-1, 2.sub.-2 in the status immediately prior to the status for which there is the possibility of the generation of trouble such as cavitation, and this value is the set value (fixed value) of a pump delivery pressure switch 9 provided to the side of the two pumps 2.sub.-1, 2.sub.-2 as shown in FIG. 36.
Then, in the unlikely event that the total delivery pressure for the pumps 2.sub.-1, 2.sub.-2 drops below this set value, this pressure drop is detected by the pump delivery pressure switch 9 and the signal S that expresses that the total delivery pressure of the pumps 2.sub.-1, 2.sub.-2 has dropped below the set value is output. Then, this signal S that is output from the pump delivery pressure switch 9 forcedly stops one of the pumps (pump 2.sub.-2 that is operating) that has continued operating without being stopped by failure, and prevents the occurrence of cavitation and other trouble due to the continued operation of the pump (pump 2.sub.-1) that continues operating.
However, when there is the pump delivery flow rate control apparatus of the conventional technology and there is control for either the degree of opening of the flow rate adjuster valve 4 or for the speed of the pump, the ultimate objective value for the degree of opening of the flow rate adjuster valve 4 or the ultimate objective value for the pump speed so that a delivery flow rate greater than the maximum allowable flow rate does not flow in the pump (pump 2.sub.-1) that did not fail when the pump (pump 2.sub.-2 for example) has failed, is a predetermined fixed value. Here, the determination of this fixed value beforehand must be performed by this so that for all operating statuses of the pump that did not fail and stop (pump 2.sub.-1), trouble such as destruction due to cavitation and rapid lowering of the delivery head (delivery pressure) of this pump due to cavitation do not occur. Because of this, the objective value for the degree of opening and the objective value for the speed must be determined to allow a sufficient surplus in consideration of the many conditions involved.
However, having such a surplus brings on problems of lowering of the operating efficiency of the pump (pump 2.sub.-1) by that amount.
More specifically, for the two pumps 2.sub.-1, 2.sub.-2 shown in FIG. 36, the normal status of the process liquid and the normal operating status for the case where one of the pumps (pump 2.sub.-2, for example) has stopped, the delivery flow rate value for the other pump (pump 2.sub.-1) obtained from the objective value (the fixed value of the adjustment valve degree of opening setting portion 11) for the degree of opening of the flow rate adjuster valve 4 becomes a value that is much lower than the maximum allowable flow rate for that pump (pump 2.sub.-1) and there is therefore the disadvantage that the difference between these two values cannot be effectively utilized.
Furthermore, this also means that the facility capacity that can be effectively used for each one pump (pump 2.sub.-1) is reduced by that amount. Therefore, when the pump (pump 2.sub.-2) has failed and stopped, the flow rate that can be sent by the pump (pump 2.sub.-1) that did not fail and stop and which is continuing operating is far less than is required for the amount of process liquid that is required by plant for power generation or for chemical processing.
Accordingly, in order to eliminate this problem, the facility capacity of the pump can be further increased or the number of pumps in the facility can be increased, thereby causing further problems.
In addition, problems such as the generation of cavitation in a pump occur not only when one pump that has been operating fails and stops so as to increase the delivery flow rate of the pump that did not fail and stop to greater than the maximum allowable flow rate, but also in the following cases.
(1) When there is a valve along the process piping on the suction side of the pump and when, due to some reason, the degree of opening of this valve is greater than the maximum degree of opening so that the pumping resistance becomes large when the process liquid flows through this process piping so that the suction pressure of the pump falls below the rated value.
(2) When the temperature of the process liquid on the suction side of the pump rises to above the rated value while the pump is operating.
However, in each of these cases (1) and (2), even when a pump flow rate control apparatus according to the previously described conventional example is used, this does not mean that the pump will not fail and stop, and so suitable pump flow rate control is not performed, and it is not possible to prevent trouble such as the generation of cavitation. This is the current situation.
Moreover, when there is a pump delivery pressure switch 9 (or a suction pressure switch) installed on the delivery side (or the suction side of the pump 2.sub.-1, 2.sub.-2, in the case (1) described above, the total delivery pressure (or the suction pressure) of the pumps 2.sub.-1, 2.sub.-2 falls below the set value and so this can be detected so that prior to the generation of cavitation, it is possible to forcedly stop a pump that is operating and therefore protect it. However, in the case (2) described above, the temperature of the process liquid on the suction side of the pump rises beforehand but the suction pressure (or the delivery pressure) does not always drop to below the set value and so even if this is done, it is not possible to prevent the generation of cavitation.
However, the judgment for whether or not trouble such as pump cavitation or the like is occurring can be performed by determining whether or not the flowing equation (1) is established for the process liquid on the suction side of the pump. EQU Ha-hr&gt;0 (1)
Where,
Ha: pump available net suction head PA1 hr: pump required net suction head PA1 D: absolute pressure applied to the liquid surface of the process liquid on the suction side of the pump; PA1 ys: height from the liquid surface of the process liquid on the suction side of the pump to the pump suction portion (a positive value when the pump suction portion is lower than the liquid portion); PA1 Zs: loss head inside pump suction piping; PA1 Pv: saturation vapor pressure of the process liquid in pump suction portion; and PA1 .gamma.: specific gravity of the process liquid on suction side of pump. PA1 H.sub.1 : pressure of the process liquid at the point of measurement; and PA1 H.sub.2 : saturation steam pressure with respect to the temperature of the process liquid at the point of measurement. PA1 k.sub.1, k.sub.2 : positive constants PA1 a: first function generator signals PA1 b: second function generator signals
Moreover, the pump available net suction head Ha described above is a value that is determined by the process piping system and the pump required net suction head hr is a value determined by the structural design of the pump and the operating conditions and the like.
The pump available net suction head Ha described above is determined by the following equation. EQU Ha=D/.gamma.+ys-Zs-Pu/.gamma. (2)
Where,
Moreover, the loss head inside pump suction piping Zs is a value that is determined by the flow rate of the process liquid that flows in the piping, and the diameter, curvature and length of the piping.
However, since there is no instrumentation to constantly and accurately measure in real time whether or not this pump available net suction head Ha can be withstood, the above equation (1) cannot be used to investigate the pump flow rate control apparatus, and so no such control apparatus exists. It is for this reason that the pump flow control apparatus that has been described above has been conventionally used.
When the pump available net suction head can be determined by equation (2) that describes the Ha and the right hand side of this equation can be thought of as follows. EQU Ha=H.sub.1 -H.sub.2 EQU H.sub.1 =D/.gamma.+ys-Zs EQU H.sub.2 Pv/.gamma.
Where,
More specifically, the pressure difference between the saturation vapor pressure with respect to the temperature of the process liquid at the point of measurement, and the pressure of the process liquid at a point of measurement can be measured.
Conventionally, an apparatus as shown in FIG. 37 is known as an apparatus for measuring this pressure difference.
More specifically, in this figure, 50 is a pressure difference transmitter, and is installed at a position separate from the process piping 49 for the purpose of improving the maintainability of the pressure difference transmitter 50 and in order to protect it from thermal transmission and vibration from the process piping and the pump.
In addition, the pressure difference sensor portion 54 of the pressure difference transmitter 50 is separated by the high-pressure side pressure-receiving portion 56 and the low-pressure side pressure-receiving portion 57. Then, the pressure of the process liquid .alpha. (alpha) inside the process piping 49 installed on the suction side of the pump, is led to the high-pressure side pressure-receiving portion 56 of the pressure difference transmitter 50 via the pressure pipe 51. On the other hand, the pressure inside the valve 52 that is inserted in the process liquid .alpha. on the suction side of the pump is lead to the low-pressure side portion side pressure-receiving portion 57 of the pressure difference transmitter 50 via a capillary tube 53.
Moreover, the valve 52, the capillary tube 53 and the inside of the low-pressure side pressure-receiving portion 57 are maintained in a state of vacuum, and to the lower portion of the valve 52 is sealed the process liquid .alpha.. More specifically, the inside of the valve 52 and the low-pressure side pressure-receiving portion 57 is made a vacuum when the pressure difference transmitter 50 is assembled, and the process liquid .alpha. is sealed inside the lower portion of the valve 52 so that the pressure of the liquid in the upper portion of the valve 52, the capillary tube 53 and the low-pressure side pressure-receiving portion 57 becomes the saturation vapor pressure at the temperature of the required flow rate set value "a", that is sealed in the bottom portion of the valve 52.
In addition, at the same time, electrical signals are also inserted to a force coil 63 and, because of this force coil 63, a force that applies a displacement of the same magnitude and the opposite direction to the previously described displacement is applied to a sensor diaphragm 55 and a force rod 60 via a mechanism 61, so that the sensor diaphragm 55 and the force rod 60 return once again to their original positions.
More specifically, by this series of actions, a differential pressure is applied to the high-pressure side pressure-receiving portion 56 and the low-pressure side pressure-receiving portion 57 and there is no displacement of the sensor diaphragm 55 but electrical signals proportional to this differential pressure are output from the amplifier 64.
Moreover, for the sake of reference, FIG. 38 shows the displacement of the saturation steam pressure with respect to each temperature for the case when the process liquid is water.
Here, the valve 52 is inserted in the process liquid .alpha. on the suction side of the pump and so thermal transmission via the wall of the valve 52 causes the temperature of the process liquid .alpha. sealed in the lower portion of the valve 52 and the temperature of the process liquid .alpha. on the suction side of the pump in a status of thermal equilibrium to become the same. In this status, the pressure of the top portion of the valve 52, the capillary tube 53 and the low-pressure side pressure-receiving portion 57 is the saturation vapor pressure at the temperature of the process liquid on the suction side of the pump.
On the other hand, the pressure of the process liquid on the suction side of the pump is led to the high-pressure side pressure-receiving portion 56 of the pressure difference transmitter 50 via the high-pressure side pressure-receiving portion 56 and the pressure difference (differential pressure) between this high-pressure side pressure-receiving portion 56 and the low-pressure side pressure-receiving portion 57 is equivalent to the pump available net suction head Ha.
However, this apparatus has the following disadvantages.
(1) The process liquid .alpha., sealed in the lower portion of the valve 52, transmits the saturation vapor pressure caused by that temperature to the low-pressure side pressure receiving portion 57; but for reasons already explained, the pressure difference transmitter 50 is installed at a position remote from the process piping 49 where the peripheral temperature is close to room temperature. Not only this, the internal diameter and the external diameter of the capillary tube 53 between the valve 52 and the low-pressure side pressure-receiving portion 57 is normally small when compared to the internal diameter of the valve 52 so as to improve the workability when the capillary tube 53 is installed so as to improve the measurement accuracy in the temperature measuring instrument where the liquid is sealed in the valve 52. Also, in this apparatus, the medium that transmits the saturation vapor pressure of the upper portion of the valve 52 is the saturated vapor inside the capillary tube 53 and the low-pressure side pressure-receiving portion 57. However, when the peripheral temperature of the low-pressure side pressure-receiving portion 57 and the capillary tube 53 installed at a position close to it is close to room temperature, the temperature of the saturated vapor in the low-pressure side pressure-receiving portion 57 and the capillary tube 53 also becomes close to room temperature and so the saturation vapor pressure in this portion also becomes the saturation vapor pressure for room temperature of the process liquid .alpha..
More specifically, there is a differential pressure in the saturation vapor which is the pressure medium in the upper portion of the valve 52, the capillary tube 53 and the low-pressure side pressure-receiving portion 57 and as a result, it is not possible to perform accurate measurements.
(2) In addition, a pressure change must be transmitted from the portion where the relative volume is small (the upper portion of the valve 52) via the restricting portion that is the capillary tube 53, to a portion where there is a large volume (the low-pressure side pressure-receiving portion 57), so the measurement error becomes even larger.
(3) As already described for (1), the process liquid .alpha., sealed in the lower portion of the valve 52, is heated by the process liquid .alpha. inside the process piping 49 and is vaporized after becoming a saturation vapor, but the portion close to the low-pressure side pressure-receiving portion 57 is near room temperature and so one portion is cooled, liquified and becomes the process liquid .alpha.. In this manner, the pressure inside the low-pressure side pressure-receiving portion 57 becomes the saturation vapor pressure of the process liquid .alpha. at a temperature close to room temperature and the pressure inside the low-pressure side pressure-receiving portion 57 is lower than the pressure in the upper portion of the valve 52, and so the saturation vapor of the process liquid supplied from the side of the valve 52 to the side of the low-pressure side pressure-receiving portion 57 always condenses inside the low-pressure side pressure-receiving portion 57 to become process liquid .alpha. and collect here. It therefore becomes impossible to apply the saturation vapor pressure with respect to the process liquid at room temperature, to the low-pressure side pressure-receiving portion 57 and ultimately, it becomes impossible to measure whether or not there is no process liquid in the bottom portion of the valve 52.
(4) Also, when this apparatus is installed, then should the valve 52 be inadvertently turned upside down or inclined and installed, the process liquid inside the valve 52 enters into and collects inside the capillary tube 53 and flows into the low-pressure side pressure-receiving portion 57 so that measurement is again rendered impossible.
Because of these disadvantages, the current situation is such that it is not possible to constantly and accurately measure in real time the degree to which the pump available net suction head can be withstood.
With respect to this, there has been disclosed in Japanese Patent Application Laid-Open Publication No. 127993-1991 (Mitsubishi Electric Corporation), a pump facility that receives first water supply flow amount control signals corresponding to a negative load, that adjusts the supply flow rate to that negative load, that is provided with a pump suction flow rate meter provided on an intake side of a pump, a first function generator that receives these signals of the suction flow rate meter and calculates the required net pump suction head (N.P.S.H.), a second function generator that receives signals from the pressure meter and the water supply temperature meter and calculates the available N.P.S.H., a controller to which the signals of the first function generator are input and subtracted and to which the signals of the second function generator are input and added and which outputs second water supply flow rate control signals, and a low signal selector that receives the output of the controller and the first water supply flow rate control signals and that sends the weaker of the two signals to a means for adjusting the water supply flow rate, so that even if there is a change in the operating status, the adjustment of the water supply flow rate to the load enables the available N.P.S.H. to always be maintained at above the required N.P.S.H. so that it is possible to prevent cavitation of the pump.
However, in the apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 127993-1991, the controller performs either proportional or proportional+integral calculation so that EQU a-k.sub.1 b-k.sub.2 .gtoreq.0
Where,
However, when there is only a proportional calculation, there is a remaining offset, so that for example, when there is control of the water supply flow rate by signals from the controller, the required N.P.S.H. actually becomes greater than the available N.P.S.H. Furthermore, when there is proportional+integral calculation, and the values of the control signals to make agreement with the required flow rate value are slightly larger than the values of the signals from the controller, the signals from the controller are saturated by integration. In this status, then even if the required N.P.S.H. is greater than the available N.P.S.H., there is no corrected signal output until the controller output due to integration becomes zero and so the required N.P.S.H. continues to be greater than the available N.P.S.H. for a long time, and as a result, it is possible that cavitation may occur.